The invention relates to a rotor of a coreless motor, and more particularly, to a rotor of coreless motor having a rotor coil which is balanced about a rotary shaft, and to a method of manufacturing same.
The requirements for a coreless motor, in particular, that one used in a small size tape recorder, are a consistent rotation at uniform rate in a smooth and stable manner free from non-uniformity in its rotation. This requires that the rotor be dynamically balanced with respect to the rotary shaft. A conventional coreless motor is manufactured by manually placing the coil in a suitable position through the use of a jig, and molding the assembly in a casting material to form the rotor. This involves extreme difficulties to achieve the desired dynamic balance.
Referring to FIGS. 1 to 3, a conventional arrangement with three coils will be described. A coreless motor 1 includes a housing 7 which is formed by a hollow cylinder 6 and a pair of upper and lower end plates 5a, 5b secured to the opposite ends of the cylinder and serving as yokes. A pair of bearings 8a, 8b are fitted centrally in each end plate 5a, 5b for rotatably receiving a rotary shaft 2. A rotor 4 is integrally coupled with the rotary shaft 2 and includes a commutator 3 and rotor coils 15a, 15b, 15c (see FIG. 6). The lower end of the shaft 2 bears against an abutment plate 9 which is secured to the end plate 5b below the bearing 8b while the upward thrust of the shaft is counteracted by the abutment of the upper end face of the commutator 3 against the lower end face of the upper bearing 8a through a spacer 13 interposed therebetween. A magnet 10 is fixedly mounted within the housing 7 so as to be opposite to the upper surface of the rotor 4, and a pair of stationary brushes 11a, 11b are fixedly mounted on the housing 6 for contact with the commutator 3.
The rotor 4 of such a conventional coreless motor is manufactured by the following steps: initially, an assembly 17 shown in FIG. 2 is prepared by bonding the rotary shaft 2 and the commutator 3 together with an adhesive 12. The lower end of the assembly 17 is formed with a flange 17a. A casting jig 14 is used to join the coils 15a to 15c with the assembly 17, and has a recess 14a formed in its upper surface which is of the same configuration as the rotor 4, with a fitting opening 14b formed centrally in the bottom of recess 14a for fitting engagement with the flange 17a of the assembly. Opening 14c receives the bottom end of shaft 2. After fitting the flange 17a into the opening 14b, the ends of the coils 15a to 15c are connected, as by soldering, with the commutator 3 and placed within the jig 14 as shown in FIG. 6. As shown in FIGS. 4 and 5, the coils are formed of conductive wires or bands without a bobbin. Then a casting material 40 such as epoxy resin, for example, is poured into the recess of jig 14, which is heated to have the assembly bonded together. When the resin is set, the entire assembly may be removed from the jig 14 to provide a rotor 4 as shown in FIG. 1 which is complete with rotary shaft 2 and commutator 3. To achieve a dynamic balance of the rotor 4 with respect to the rotary shaft 2, the coils 15a to 15c must be disposed within the jig 14 so as to maintain an equal spacing between them and a uniform distribution. Because the disposition is a manual operation, extreme care must be exercised in positioning the coils while avoiding any movement thereof, requiring a considerable length of time. Still, the positioning cannot be satisfactory, causing an imbalance to the rotor which presents an impediment to the smooth rotation of the rotor.
To overcome such difficulty, there is proposed the use of guide pins secured to the interior of the jig and which can be directly engaged with the coils to position them. However, this renders the release of the molded assembly from the jig difficult since the resin which extends into the coils is adhesively bonded with the pins. In addition, if the next set of rotor coils are attached to the guide pins before the jig cools down from the heating step used to set the resin, the coating of the coil winding may be damaged by the guide pins.